Under circumstances where it is necessary to separate and measure the concentrations of components of a mixture, one commonly used instrument is a gas chromatograph. In such instruments, a sample in the vapor phase is injected into a carrier gas stream which transports the sample to a chromatographic column. Within this column the carrier gas continues to pass at a uniform rate while the components of the sample are retained at rates that depend upon a number of factors. Understanding these factors is a study beyond the scope of this invention. The differences in these retention values cause the various components of the sample to separate into bands which travel through the column at characteristic rates. This, in turn, allows the distribution of the components to be determined from the location and size of the bands.
In order to increase the rate at which samples of test liquids may be processed through a gas chromatograph, it is customary to use a device known as a liquid sample injection valve. The latter valve automates the process of taking a sample of the test liquid and delivering the same for vaporization in the presence of the carrier gas stream of the gas chromatograph. Valves of this type typically include a pneumatically driven metering rod that reciprocates between a low temperature sample loading assembly and a high temperature sample evaporating assembly.
Since the metering rod carries a sample in a sample cavity or groove having a known volume, it is important that the temperature of the metering rod be held constant. This is because temperature differences can cause the density of the sample to vary resulting in erroneous concentration readings. Additionally, it is desirable to keep the sample flowing past the sample cavity or groove at a low temperature to reduce the danger of handling flammable liquids, to comply with the law (e.g. Germany), and to avoid formulation of bubbles at the sample cavity or groove which would affect sample size. It follows, then, that neither the proximity of the low temperature sample loading assembly, nor the reciprocation of the metering rod therebetween, can be allowed to affect the maintenance of a constant temperature at the sample cavity or groove which is substantially lower than that of the sample evaporation assembly.
In attempting to meet the foregoing requirements, various types of liquid sample injection valves have been developed. One of these valves is described in U.S. Pat. No. 3,401,565 issued on Sept. 17, 1968 in the name of E. H. Stoll et al. The latter patent describes a valve in which the desired thermal isolation between the sample loading and evaporating assemblies is provided by connecting these assemblies through a relatively long and narrow neck that is wrapped with cooling coils which carry a flow of a liquid coolant. While this structure provided the desired thermal isolation, it also resulted in the need for relatively long fluid seals for the metering rod, and in the concentration of excessive mounting stresses in vicinity of the connecting neck. Other problems with such valves included the presence of dead volumes, i.e., spaces within which portions of a sample could accumulate and contaminate subsequently injected samples, and the difficulty of adjusting the pressure on or replacing the seals associated with the metering rod.
Another type of liquid sample injection valve is described in U.S. Pat. No. 3,643,511 issued on Feb. 22, 1972 in the name of Warncke et al. In valves of the latter type the desired thermal isolation between the sample loading and evaporating assemblies was afforded by constructing the sample chamber housing from a flexible insulating material. In addition, the pressure on the seals associated with the metering rod was made adjustable by compressing the entire sample chamber housing. One problem with this structure was that the compression of the sample chamber housing induced therein stresses that could in time result in leaks between the sample chamber housing and the inlet and outlet fittings associated therewith. Another problem was that, like the valve described in the Stoll patent, the valve of the Warncke patent included a narrow neck which provided only a weak structural connection between the sample loading and evaporating assemblies.